Water-in-oil emulsion type blasting agents were first disclosed by Bluhm in U.S. Pat. No. 3,447,978. These emulsion type blasting agents contain an aqueous solution of inorganic oxidizer salts that is emulsified as the dispersed phase within a continuous carbonaceous fuel phase, and a uniformly distributed gaseous component. Later, cap sensitive emulsion explosive compositions were produced using explosive additives such as trinitrotoluene, and pentaerythritol tetranitrate, (see e.g., U.S. Pat. No. 3,770,522). Water-in-oil emulsion explosive compositions have also been made cap sensitive by the addition of nonexplosive detonation catalysts (see e.g., U.S. Pat. Nos. 3,715,247 and 3,765,964). Most recently, cap sensitive water-in-oil emulsion type explosive compositions, containing neither explosive ingredients nor detonation catalysts, have been disclosed in U.S. Pat. Nos. 4,110,134, 4,149,916 and 4,149,917.
While the cap sensitive emulsion explosive compositions disclosed in the above-identified patents satisfy a wide range of requirements, there are certain blasting applications in which even higher sensitivities than are available using such compositions would be advantageous. Specifically, in coal mining where it is especially important that a string of cartridges be able to propagate the explosion from one cartridge to the next, 1/2 cartridge gap sensitivity tests are used to determine the suitability of the explosive for use in such applications. Basically, this test measures sensitivity in terms of the length of the air gap across which one half of a standard cartridge (11/4" by 8" in length) of explosive material can detonate a second half of a cartridge. Thus, for example, the preferred cap sensitive emulsion explosive composition as prepared according to the disclosures of U.S. Pat. No. 4,110,134, have an air gap sensitivity of about two inches. As noted above, cap sensitive compositions having sensitivities greater than those of heretofore available cap sensitive emulsion explosive compositions are desirable in certain blasting applications.